Laterally insertable artificial vertebral disk replacement implant with crossbar spacer

ABSTRACT

The present invention is directed to an implant that can be placed between two adjacent vertebral bodies using a lateral insertion method. The implant is characterized as having a first end plate and a second end plate which a crossbar spacer therebetween. The crossbar spacer preferably fits within a channel on the inner surfaces of the first end plate and the second end plate, whereby the crossbar spacer allows pivots, twisting and/or rotational movement of the spine. The first end plate and the second end plate include a keel extending therefrom, whereby the keel traverses longitudinally between a first lateral side and a second opposed lateral side and is substantially perpendicular to the sagittal plane of the patient&#39;s spine.

CLAIM OF PRIORITY

This application claims priority under 35 USC 119 to U.S. PatentApplications No. 60/517,791, filed Nov. 5, 2003 and entitled “ARTIFICIALVERTEBRAL DISK REPLACEMENT IMPLANT WITH CROSSBAR SPACER AND LATERALIMPLANT METHOD,” and U.S. No. 60/517,973, filed Nov. 6, 2003 andentitled “ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH CROSSBARSPACER AND LATERAL IMPLANT METHOD,” both of which are herebyincorporated by reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. Provisional Application No.60/422,039, filed Oct. 29, 2002, entitled “ARTIFICIAL VERTEBRAL DISKREPLACEMENT IMPLANT WITH TRANSLATING PIVOT POINT AND METHOD,” U.S.patent application Ser. No. 10/684,668, filed Oct. 14, 2003, entitled“ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH CROSSBAR SPACER ANDMETHOD,” U.S. patent application Ser. No. 10/684,669, filed Oct. 14,2003, entitled “ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITHTRANSLATING PIVOT POINT AND METHOD,” U.S. Provisional Application No.60/422,011, filed Oct. 29, 2002, entitled “TOOLS FOR IMPLANTING ANARTIFICIAL VERTEBRAL DISK AND METHOD,” U.S. patent application Ser. No.10/685,134, filed Oct. 14, 2003, entitled “TOOLS FOR IMPLANTING ANARTIFICIAL VERTEBRAL DISK AND METHOD,” U.S. Provisional Application No.60/422,022, filed Oct. 29, 2002, entitled “ARTIFICIAL VERTEBRAL DISKREPLACEMENT IMPLANT WITH A SPACER AND METHOD,” U.S. ProvisionalApplication No. 60/422,021, filed Oct. 29, 2002, entitled “ARTIFICIALVERTEBRAL DISK REPLACEMENT IMPLANT WITH CROSSBAR SPACER AND METHOD,”U.S. patent application Ser. No. 10/685,011, filed Oct. 14, 2003,entitled “ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH SPACER ANDMETHOD,” U.S. patent application Ser. No. 10/981,863, filed Nov. 5, 2004entitled “LATERALLY INSERTABLE ARTIFICIAL VERTEBRAL DISK REPLACEMENTIMPLANT WITH TRANSLATING PIVOT POINT,” U.S. patent application Ser. No.10/981,807, filed Nov. 5, 2004, entitled “METHOD OF LATERALLY INSERTINGAN ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH TRANSLATING PIVOTPOINT,” U.S. patent application Ser. No. 10/981,952, filed Nov. 5, 2004,entitled “METHOD OF LATERALLY INSERTING ARTIFICIAL VERTEBRAL DISKREPLACEMENT IMPLANT WITH A CROSSBAR SPACER,” U.S. patent applicationSer. No. 10/981,923, filed Nov. 5, 2004, entitled “LATERALLY INSERTABLEARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH CURVED SPACER,” U.S.patent application Ser. No. 10/981,945, filed Nov. 5, 2004, entitled“METHOD OF LATERALLY INSERTING AN ARTIFICIAL VERTEBRAL DISK REPLACEMENTIMPLANT WITH CURVED SPACER,” all of which are incorporated herein byreference.

FIELD OF THE INVENTION

The field of art of this disclosure is directed to an artificialvertebral disk replacement and method.

BACKGROUND OF THE INVENTION

The spinal column is a biomechanical structure composed primarily ofligaments, muscles, vertebrae and intervertebral disks. Thebiomechanical functions of the spine include: (1) support of the body,which involves the transfer of the weight and the bending movements ofthe head, trunk and arms to the pelvis and legs, (2) complexphysiological motion between these parts, and (3) protection of thespinal cord and nerve roots.

As the present society ages, it is anticipated that there will be anincrease in adverse spinal conditions which are characteristic of aging.For example, with aging comes an increase in spinal stenosis (including,but not limited to, central canal and lateral stenosis), and facet jointdegeneration. Spinal stenosis typically results from the thickening ofthe bones that make up the spinal column and is characterized by areduction in the available space for the passage of blood vessels andnerves. Facet joint degeneration results from the constant load borne bythe facet joints, and the eventual wear that results. Pain associatedwith both conditions can be relieved by medication and/or surgery.

In addition to spinal stenosis and facet joint degeneration, theincidence of damage to the intervertebral disks is also common. Theprimary purpose of the intervertebral disk is to act as a shockabsorber. The disk is constructed of an inner gel-like structure, thenucleus pulposus (the nucleus), and an outer rigid structure comprisedof collagen fibers, the annulus fibrosus (the annulus). At birth, thedisk is 80% water which then gradually diminishes with time, therebybecoming stiff. With age, disks may degenerate, and bulge, thin,herniate, or ossify. Additionally, damage to disks may occur as a resultdisease, trauma or injury to the spine.

The damage to disks may call for a range of restorative procedures. Ifthe damage is not extensive, repair may be indicated, whereas extensivedamage may indicate full replacement. Regarding the evolution ofrestoration of damage to intervertebral disks, rigid fixation proceduresresulting in fusion are still the most commonly performed surgicalintervention. However, trends suggest a move away from such procedures.Currently, areas evolving to address the shortcomings of fusion forremediation of disk damage include technologies and procedures thatpreserve or repair the annulus, that replace or repair the nucleus, andthat advance implants for total disk replacement. The trend away fromfusion is driven both by issues concerning the quality of life for thosesuffering from damaged intervertebral disks, as well as responsiblehealth care management. These issues drive the desire for proceduresthat are minimally invasive, can be tolerated by patients of all ages,especially seniors, and can be performed preferably on an out-patientbasis.

Most recently, there has been an increased interest in total diskreplacement technology. A number of artificial disks are beginning toappear in the medical device marketplace. These artificial disks varygreatly in shape, design and functionality. With these devices areavailable tools and methods for insertion of the devices between thevertebrae.

Accordingly, there is a need in the art for innovation in technologiesand methods that advance the art in the area of minimally invasiveintervertebral disk replacement. This not only enhances the quality oflife for those suffering from the condition, but is responsive to thecurrent needs of health care management.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a front view of an embodiment of the disclosed implant.

FIG. 1B is a side view of an embodiment of the disclosed implant.

FIG. 1C is a top view of an embodiment of the disclosed implant.

FIG. 1D is a plan view of an embodiment of the first inner surface ofthe upper end plate of the disclosed implant.

FIG. 1E is a perspective view of an embodiment of the upper end plate ofthe disclosed implant.

FIG. 1F is a plan view of the second inner surface of the lower endplate of an embodiment of the disclosed implant.

FIG. 1G is a perspective view of the lower end plate of an embodiment ofthe disclosed implant.

FIG. 1H is a cross-sectional view of the upper and lower end plates ofthe implant taken along line H-H in FIG. 1A.

FIG. 2A is a top view of a crossbar of an embodiment of the disclosedimplant.

FIG. 2B is a side view of the crossbar of an embodiment of the disclosedimplant.

FIG. 2C is a bottom view of the crossbar of an embodiment of thedisclosed implant.

FIG. 2D is a perspective view of the crossbar of an embodiment of thedisclosed implant.

FIG. 3 is a perspective view of the assembled implant of an embodimentof the present invention.

FIG. 4 is a side view of an embodiment of the disclosed implantimplanted between adjacent vertebral bodies.

FIG. 5 is a block diagram showing the method of the lateral implantationof an embodiment of the disclosed the disclosed implant.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The following description is presented to enable any person skilled inthe art to make and use the implant of the present invention. Variousmodifications to the embodiments described will be readily apparent tothose skilled in the art, and the principles defined herein can beapplied to other embodiments and applications without departing from thespirit and scope of what is disclosed and defined by the appendedclaims. Thus, what is disclosed is not intended to be limited to theembodiments shown, but is to be accorded the widest scope consistentwith the principles and features disclosed herein. To the extentnecessary to achieve a complete understanding of what is disclosedherein, the specification and drawings of all patents and patentapplications cited in this application are incorporated herein byreference.

FIG. 1A shows a front view of an embodiment of the implant 100. Thedesignations, “A” for anterior, “P” for posterior, “RL” for rightlateral, and “LL” for left lateral are given in the drawings for spatialdirectional orientation. These designations give the relationship of allfaces or ends of the implant with respect to the superior perspective;i.e. looking down the axis of the spine. As shown in the Figures, theimplant 100 has an anterior side A, a posterior side P, and two lateralsides, LL and RL which extend between the anterior side A and theposterior side P. The anterior side A faces the anterior direction whenthe implant 100 is inserted into the spine. The posterior side P facesthe posterior direction when the implant 100 is inserted into the spine.The right lateral side RL faces the lateral direction away from theright side of the spine, whereas the left lateral side LL faces thelateral direction away from the left side of the spine when the implant100 is inserted therein. In one embodiment, as shown in FIG. 1C, theimplant 100 has a perimeter shape which is configured to correspond tothe perimeter shape of the vertebral disks, whereby the anterior side Ais curved and the posterior side P is parallel to the sagittal plane. Inanother embodiment, the implant 100 has a perimeter shape which does notcontour to the shape of the vertebral disks. As will be appreciated bythose of ordinary skill in the art, the perimeter shape of the upper endplate 110 and the lower end plate 120 can be the same or different fromone another.

The implant 100 preferably includes an upper end plate 110 that isconfigured to mate with an upper vertebral body. The implant 100preferably includes a lower end plate 120 that is configured to matewith a lower vertebral body. The implant 100 also includes a spacer 130positioned between the upper end plate 110 and the lower end plate 120.The spacer 130 separates the upper end plate 110 from the lower endplate 120 and also facilitates pivotal and/or rotational as well astwisting movement of the upper end plate 110 and the lower end plate 120relative to each other and the spacer 130. The spacer 130 is preferablyin the form of a crossbar as discussed in more detail below.

The upper end plate 110 has a first outer surface 112 which comes intocontact and mates with the underside of the upper vertebral body. Theimplant 100 includes a first keel 114, as shown in FIGS. 1A-1C, whichpreferably extends away from the first outer surface 112 in a directionsubstantially perpendicular to the first outer surface 112. In oneembodiment, when the implant 100 is inserted between the vertebralbodies, the first keel 114 preferably extends into a keel receivingchannel which is cut into the underside of the upper vertebral body toanchor the upper end plate 110 thereto. The first keel 114 extendslongitudinally across the first outer surface 112 between the leftlateral side LL and the right lateral side RL, as shown in FIG. 1C. Thefirst keel 114 is thus oriented to be substantially perpendicular to thesagittal plane of the spine, which is known to one skilled in the art asthe plane which traverses from the posterior toward the anterior of thepatient or patient's spine. As shown in FIG. 1C, it is preferred thatthe right lateral side RL and left lateral side LL of the implant 100are parallel to the sagittal plane. In a preferred embodiment, the firstkeel 114 is oriented to be substantially parallel to the coronal planeof the spine which is known in the art as the plane that is parallel tothe patient's shoulders. In another embodiment, the first keel 114extends longitudinally only partially across the first outer surface112. It should be noted that although one keel 114 is shown laterallyacross the first outer surface 112 in FIG. 1C, more than one keel 114 isalternatively disposed on the first outer surface 112, as shown in FIG.4. It is contemplated that the plurality of keels 114, as shown in FIG.4, are parallel to each other and are perpendicular to the sagittalplane of the spine, although not necessarily. Although the implant ofthe present invention is shown and described herein as having one ormore keels laterally oriented between the right and left lateral sides,the implant described herein can alternatively have keels which areoriented between the anterior and posterior sides as described in U.S.patent application Ser. No. 10/684,668 which is incorporated byreference.

It is preferred that the first keel 114 of the upper end plate 110includes a plurality of teeth 115. In one embodiment, the teeth 115 ofthe keel 114 are angled and point towards the left lateral face of theimplant 100, as shown in the example in FIG. 1A. The left angled teeth115 allow the upper end plate 110, as shown in FIG. 1A, to be easilyinserted between the vertebral bodies from a left lateral approach. Thisis so, because the angled configuration of the teeth 115 provide littlefrictional resistance against the underside of the vertebral body wheninserted into the spine. The angled configuration of the teeth 115 alsoprevents the upper end plate 110 from becoming dislodged orunintentionally slipping out of the spine after being inserted therein.In another embodiment, the teeth 115 of the keel 114 are angled andpoint towards the right lateral face RL of the implant 100, opposite ofthat shown in the embodiment in FIG. 1A. This configuration allows theupper end plate 110 to be easily inserted into the spine from a rightlateral side approach.

The lower end plate 120 of the present implant 100 includes a secondouter surface 122, as shown in FIGS. 1A and 1B. The second outer surface122 preferably comes into contact with and mates with the top-side ofthe lower vertebral body. The implant 100 includes a second keel 124, asshown in FIGS. 1A and 1B, which preferably extends away from the secondouter surface 122 in a direction substantially perpendicular to thesecond outer surface 122. In one embodiment, when the implant 100 isinserted between the vertebral bodies, the second keel 124 preferablyextends into a keel receiving channel in the top-side of the lowervertebral body to anchor the lower end plate 120 thereto. The secondkeel 124 extends longitudinally across the second outer surface 122between the left lateral side LL and the right lateral side RL, as shownin FIG. 3. In another embodiment, the second keel 124 extendslongitudinally only partially across the second outer surface 122. Thesecond keel 124 is thus oriented to be substantially perpendicular tothe sagittal plane of the spine. In a preferred embodiment, the secondkeel 124 is oriented to be substantially parallel to the coronal planeof the spine. It should be noted that although one keel 124 is shownlaterally across the second outer surface 122 in FIG. 3, more than onekeel 124 is alternatively disposed on the second outer surface 122, asshown in FIG. 4. It is contemplated that the plurality of keels 124, asshown in FIG. 4, are parallel to each other and are perpendicular to thesagittal plane of the spine.

It is preferred that the second keel 124 of the lower end plate 120includes a plurality of teeth 125. In one embodiment, the teeth 125 ofthe keel 124 are angled and face towards the left lateral face of theimplant 100, as shown in the example in FIG. 1A. The left angled teeth125 allow the lower end plate 120, as shown in FIG. 1A, to be easilyinserted between the vertebral bodies from a left lateral approach. Thisis so, because the angled configuration of the teeth 125 provide littlefrictional resistance against the top-side of the vertebral body wheninserted into the spine. The angled configuration of the teeth 125 alsoprevents the lower end plate 120 from becoming dislodged orunintentionally slipping out of the spine after being inserted therein.In another embodiment, the teeth 125 of the keel 124 are angled andpoint towards the right lateral face RL of the implant 100, opposite ofthat shown in the embodiment in FIG. 1A. This configuration allows thelower end plate 120 to be easily inserted into the spine from a rightlateral side approach.

In the embodiment shown in FIG. 1A, the first and second keels 114,124preferably include ports 148, 152 therethrough. The ports 148, 152facilitate bone in-growth, wherein bone from the vertebral bodies cangrow thorough the ports 148,152 to aid in securing the first and secondkeels 114, 124, and thus the implant 100 to the spine. In one embodimentthe outer surfaces of the first and second keels 114,124 and the firstand second outer surfaces 112, 122 of the implant 100 are roughened inorder to promote bone in-growths into the defined roughened surfaces ofthe implant 100. In one embodiment, the first and second keels 114,124,and the first and second outer surfaces 112, 122 of implant 100 arecoated with one or more materials that promote bone growth. Suchmaterials include, but are not limited to, bone morphogenic protein(BMP) and hyaluronic acid. Other substances which promote bone growthrelative to and into the keel, keel ports, and other external surfacesof the implant 100 are contemplated.

The first and second keels 114, 124 preferably extend between thevertebral bodies to anchor the implant 100 to the spine. The lateralorientation of the first keel 114 and the second keel 124 allow theimplant 100 to be inserted into the spine using a lateral approach asopposed to an anterior or posterior approach, which is advantageous,because the spinal nerves in the spinal cavity are minimally undisturbedwhen the implant 100 is inserted laterally. In comparison to a posteriorinsertion approach in which the spinal nerves can be substantiallydisturbed, the spinal nerves are bypassed and relatively undisturbedwhen the implant 100 is inserted laterally between the vertebral bodiesfrom the side of the spine. Although an anterior insertion approach hasits benefits, the lateral insertion approach can allow the presentimplant 100, and associated implantation tools, to be inserted into thespine with less disturbance of the patient's internal organs. This cantranslate into less time and risk associated with preparing the spinefor insertion as well as inserting the implant itself into the spine.Further, the laterally oriented first and second keels 114, 124 offersubstantial stability to the vertebral bodies during extension, flexionand lateral bending of the spine.

As shown in FIGS. 1D and 1E, the upper end plate 110 includes a firstinner surface 116. It should be noted that the upper end plate 110 isshown oriented upside down in FIGS. 1D and 1E, whereby the inner surface116 is shown facing upwards. The first inner surface 116 receives andengages the spacer 130 of the implant and opposes an inner surface 126(FIG. 1B) of the second end plate 120. In one embodiment, the firstinner surface 116 is designed to form a planar surface that is parallelwith the first outer surface 112. In another embodiment, the first innersurface 116 is designed to form a planar surface that is non-parallelwith the first outer surface 112. In particular, as shown in FIGS. 1Band 1E, the anterior side A of the end plate 110 has a larger thickness(i.e. distance between the first outer side 110 and first inner side116) than the thickness of the posterior side P.

As shown in FIGS. 1D and 1E, the first inner surface 116 of the upperend plate 110 preferably includes a channel, also referred to as asocket, 150 therein. As will be discussed below, the spacer includes aspacer beam, wherein at least a portion of the spacer beam can be seatedin the channel 150 to allow the first and/or second end plates of theassembled implant 100 to pivot or rotate relative to each other. Asshown in FIGS. 1D and 1E, the channel 150 is preferably concave andextends lengthwise between the left lateral side LL and the rightlateral side RL of the upper end plate 110. Alternatively, the channel150 extends lengthwise between the anterior side A and the posteriorside P of the upper end plate 110. In one embodiment, as shown in FIG.1E, the upper end plate 110 includes a ridge 117 formed in the firstinner surface 116, whereby the ridge 117 is surrounded by the firstinner surface 116, which is raised relative thereto. In one embodiment,the channel 150 receives a portion of the spacer beam. In anotherembodiment, the spacer beam fits within the channel 150.

As shown in FIGS. 1F and 1G, the lower end plate 120 includes a secondinner surface 126. The second inner surface 126 receives and engages thespacer 130 of the implant and opposes the inner surface 126 (FIG. 1E) ofthe first end plate 110. In one embodiment, the second inner surface 126is designed to form a planar surface that is parallel to the secondouter surface 122. In another embodiment, the second inner surface 126is designed to form a planar surface that is non-parallel to the firstouter surface 122. In particular, as shown in FIGS. 1G and 1E, theanterior side A of the end plate 120 has a smaller thickness (i.e.distance between the second outer side 120 and second inner side 126)than the thickness of the posterior side P.

As shown in FIGS. 1G and 1H, the second inner surface 126 of the lowerend plate 120 preferably includes a channel, also referred to as asocket, 160 therein. As will be discussed below, the spacer 130 includesa spacer beam which can be placed into the channel 160 in order to allowthe first and/or second end plates of the assembled implant to pivot orrotate relative to each other. As shown in FIGS. 1G and 1H, the channel160 is preferably concave and extends lengthwise between the anteriorside A and the posterior side P of the lower end plate 120.Alternatively, the channel 160 extends lengthwise between the leftlateral side LL and the right lateral side RL of the lower end plate120. In one embodiment, as shown in FIG. 1G, the lower end plate 120includes a ridge 127 formed in the second inner surface 126, whereby theridge 127 is surrounded by the second inner surface 126, which is raisedrelative thereto. In one embodiment, the channel 160 receives a portionof the spacer beam. In another embodiment, the spacer beam fits withinthe channel 160.

In one embodiment, the first outer surface 112 of the first end plate110 is substantially parallel to the second outer surface 122 of thesecond end plate 120 when the implant 100 is assembled and is in aneutral position (i.e., the position where the first end plate 110 hasnot rotated relative to the second end plate 120). Alternatively, thefirst outer surface 112 of the first end plate 110 is non-parallel tothe planar surface of the second outer surface 122 of the second endplate 120 when the implant 100 is assembled and in the neutral position.The non-parallel orientation of the first end plate 110 and the secondend plate 120 allows the plates to pivot a greater degree with respectto each other. Additionally, other factors such as the height of thespacer 130 and the position of the keel receiving channels can beadjusted in order to increase the degree by which the first end plate110 and the second end plate 120 can pivot relative to each other andthe spacer 130.

FIG. 1H illustrates a cross-section of the implant 100 taken along thelines H-H of FIG. 1A. As shown in FIG. 1H, the first inner surface 116of the first plate 110 substantially opposes the second inner surface126 of the second plate 120. In addition, FIG. 1H illustrates the firstchannel 150 located in the first inner surface 116 which extendslaterally between the left lateral side LL and the right lateral sideRL. Additionally, the second channel 160 is located in the second innersurface 126 and extends between the anterior side A and the posteriorside P, as shown in FIG. 1H.

In one embodiment, the planar surfaces which correspond to the first andsecond outer surfaces 112, 122 of the implant lie parallel orsubstantially parallel to the axial plane of the body when the implant100 is inserted between adjacent vertebrae. In one embodiment, theplanar surfaces which correspond to the first and second inner surfaces116, 126 of the first and second end plates 110, 120 lie parallel, orsubstantially parallel, to the axial plane of the body when the implantis implanted. In one embodiment, the outer surfaces 112, 122 and theinner surfaces 116, 126 both lie parallel or substantially parallel tothe axial plane of the body when the implant is implanted. In each ofthe embodiments, either or both keels 114, 124 are perpendicular to thesagittal plane. Preferably, the keels 114, 124 are also parallel to thecoronal plane of the body.

FIG. 2A illustrates a top view of the preferred crossbar spacer 130 ofthe present invention. The crossbar spacer 130 preferably has a firstspacer beam 210 and a second spacer beam 220 as shown in FIG. 2A,whereby the first spacer beam 210 is oriented perpendicular to thesecond spacer beam 220. The first spacer beam 210 has a first end 212and a second opposed end 214 as well as a midpoint 216 therebetween. Thesecond spacer beam 220 has a first end 222 and a second opposed end 224as well as a midpoint 226 therebetween. As shown in the figures, thespacer beams 210, 220 are preferably circular in cross section, althoughother appropriate shapes are contemplated.

In the embodiment shown in FIG. 2A, the midpoint 216 of the first spacerbeam 210 is not aligned with the midpoint 226 of the second spacer beam220. Instead, the midpoint 216 of the first spacer beam is proximal tothe first end 222 and distal to the second end 224 of the second spacerbeam 220. In this configuration, the first and second beams 210, 220substantially form a cross or “T” shape.

in another embodiment, the first beam 210 can be positioned transverselyalong the length of the second beam 220 such that the midpoint 216 ofthe first beam 210 and the midpoint 226 of the second beam 220 arealigned with one another. Where both beams 210, 220 are positioned atthe respective midpoints 216, 226, the crossbar spacer 130 substantiallyforms a plus sign, “+”.

FIG. 2B shows a side view of the crossbar spacer 130 of the implant 100.As is apparent from FIG. 2B, the first beam 210 is oriented such thatthe center 213 of the first beam 210 is off-set from the center 223(FIG. 2C) of the second beam 220. Thus, the first beam 210 is locatedalong a plane which is parallel and adjacent to a plane along which thesecond beam 220 is oriented. Similarly, as shown in FIG. 2C, the secondbeam 220 is oriented such that the center 223 of the first beam 220 isoff-set from the center 213 of the first beam 210. The off-setpositioning of the first beam 210 and the second beam 220 allows thespacer 130 to be positioned between, and in contact with, the upper endplate 110 and the lower end plate at the same time. Additionally, theoff-set positioning of the beams 210, 220 allow the upper and lower endplates 110, 120 to pivot about the beams of the spacer 130 toaccommodate flexion, extension, twisting and/or lateral bending of thespine.

In constructing the crossbar 130, the first beam 210 can be formedintegrally with the second beam 220 to be unitary in construction.Alternatively, the first beam 210 can be adhered to the second beam 220using any other suitable method (e.g. spot welding). It should be notedthat the first beam 210 and the second beam 220, as well as the spacer130 as a whole, are sufficiently constructed to withstand the loadforces applied by the end plates 110, 120 in the neutral position aswell as during flexion, extension and/or lateral bending movements. Thespacer 130 can be formed by extrusion, injection, compression molding,machining or any other appropriate techniques.

Viewing FIGS. 1A, 1B and 3, an assembled embodiment of the implant 100is depicted. The upper end plate 110 is configured to mate with thefirst vertebra and the lower end plate 120 is configured to mate with asecond vertebra. A crossbar 130 that sits between the first end plate110 and the second end plate 120 is also provided. As is evident fromthe figures, the upper beam 210 of the crossbar 130 is placed in thechannel 150 of the upper end plate 110 such that the upper beam 210 isapproximately parallel with the keels 114, 124. As can be seen fromFIGS. 1H and 4, the “T”-shaped spacer 130 sits between the upper andlower end plates 110, 120, wherein the first beam 210 is preferablyreceived in the channel 150 (FIG. 1H) and the second beam 220 ispreferably received in the channel 160 (FIG. 1H). As shown in FIG. 4,the first beam 210 is positioned proximal to the posterior side P ratherthan the anterior side A of the implant 100, whereby the first beam 210is oriented parallel to the keels 114, 124 and perpendicular to thesagittal plane. In another embodiment, the upper beam 210 can bepositioned midway between the posterior and the anterior faces of theimplant 100 in the embodiment that the crossbar spacer 130 has a “+”shape. As shown in FIG. 4, the lower beam 220 is placed in the channel160 of the lower end plate 120 such that the lower beam 220 isapproximately perpendicular to the keels 114, 124 and parallel with thesagittal plane.

As stated above, the crossbar spacer facilitates pivotal or rotationalmovement of the first end plate 110 and the second end plate 120,relative to each other. In particular, the interface between the firstbeam 210 of the spacer 130 and the channel 150 allows the upper endplate 110 and/or lower end plate 120 to pivot or rotate about the firstbeam 210 when the patient moves backwards (extension) and forwards(flexion). Additionally, the interface between the second beam 210 ofthe spacer 130 and the channel 160 allows the lower end plate 120 and/orupper end plate 110 to pivot or rotate about the second beam 210 whenthe patient bends side to side (laterally). The patient's weight as wellas gravity hold the spacer 130 securely seated within the channels 150,160. As stated above, the implant 100 includes channels 150, 160 in oneembodiment, although not necessarily.

In one embodiment, there is a loose fit between the spacer 130 and theupper and lower end plates 110,120. In particular, there is a loose fitbetween the upper beam 210 and the upper channel 150 and also betweenthe lower beam 220 and the lower channel 160. This loose fit allows fora twisting motion of the upper and/or lower end plates, and thusvertebral bodies, about a vertical axis through the center of the spine.

FIG. 4 shows a side view of an implant 100 inserted between twovertebral bodies 410, 420. The implant 100 is shown inserted between thetwo vertebrae 410, 420 with two first keels 114 extending from the firstend plate 110, and two second keels 124 extending from the second endplate 120. The first and second keels 114,124 are approximatelyperpendicular to the sagittal plane of the spine, and preferablystraddle the point of articulation of the spacer 130. A gap is presentbetween the first end plate 110 and the second end plate 120 at theanterior A face of implant 100 is preferably greater than at theposterior end P face of implant 100. The larger gap at the anterior facevs. the posterior face of implant 100 allows forward (flexion) movementto be facilitated to a greater degree than backward (extension)movement. Thus, in one example of a forward bending movement of up to10° can be achieved while a backward bending movement of 5° can beachieved. Other angles are contemplated within the scope of the presentinvention.

In one embodiment, the implant can be made of medical grade titanium,stainless steel or cobalt chrome. The material has appropriate physicaland mechanical properties and is suitable for carrying and spreading thephysical load between the spinous process. Other materials that haveappropriate structural strength and that are suitable for implantationinto a patient can also be used. One class of materials contemplated foruse in implant 100 is the class of biocompatible polymers. Copolymers,blends and composites of polymers are also contemplated for fabricationof parts of the disclosed device. A copolymer is a polymer derived frommore than one species of monomer. A polymer composite is a heterogeneouscombination of two or more materials, wherein the constituents are notmiscible, and therefore exhibit an interface between one another. Apolymer blend is a macroscopically homogeneous mixture of two or moredifferent species of polymer.

One group of biocompatible polymers are the polyarylesterketones whichhas several members, which include polyetheretherketone (PEEK), andpolyetherketoneketone (PEKK). PEEK has proven as a durable material forimplants, as well as meeting criteria of biocompatibility. Medical gradePEEK is available from Victrex Corporation under the product namePEEK-OPTIMA. Medical grade PEKK is available from Oxford PerformanceMaterials under the name OXPEKK, and also from CoorsTek under the nameBioPEKK. Other materials that can be used include polyetherketone (PEK),polyetherketoneether-ketoneketone (PEKEKK), andpolyetheretherketoneketone (PEEKK), and, generally, apolyaryletheretherketone. Further, other polyketones can be used as wellas other thermoplastics.

Reference to appropriate polymers that can be used in the spacer can bemade to the following documents, all of which are incorporated herein byreference. These documents include: PCT Publication WO 02/02158 A1,dated Jan. 10, 2002, entitled “Bio-Compatible Polymeric Materials;” PCTPublication WO 02/00275 A1, dated Jan. 3, 2002, entitled “Bio-CompatiblePolymeric Materials;” and, PCT Publication WO 02/00270 A1, dated Jan. 3,2002, entitled “Bio-Compatible Polymeric Materials.”

Still another interesting group of biocompatible polymers are polyalkylbiocompatible polymers, such as polyethylenes, polypropylenes, and thelike. These medical grade biocompatible polymers are also available asreinforced polymer materials. To reinforce a polymeric material,fillers, are added to a polymer, copolymer, polymer blend, or polymercomposite. Fillers are added to modify properties, such as mechanical,optical, and thermal properties. In this case, fillers, such as carbonfibers, are added to reinforce the polymers mechanically to enhancestrength for certain uses, such as load bearing devices.

For example, other grades of PEEK are also available and contemplated,such as 30% glass-filled or 30% carbon-filled, provided such materialsare cleared for use in implantable devices by the FDA, or otherregulatory body. Glass-filled PEEK reduces the expansion rate andincreases the flexural modulus of PEEK relative to that which isunfilled. The resulting product is known to be ideal for improvedstrength, stiffness, or stability. Carbon-filled PEEK is known toenhance the compressive strength and stiffness of PEEK and lower itsexpansion rate. Carbon-filled PEEK offers wear resistance and loadcarrying capability.

Alternatively, the spacer 130 can be made out of a polymer, and morespecifically, the polymer is a thermoplastic with the other componentsmade of the materials specified above. Still more specifically, thematerial is PEEK 450G, which is an unfilled PEEK approved for medicalimplantation available from Victrex of Lancashire, Great Britain.(Victrex is located at www.matweb.com or see Boedeker www.boedeker.com).Other sources of this material include Gharda located in Panoli, India(www.ghardapolymers.com). Further in this embodiment, the PEEK has thefollowing additional approximate properties:

Property Value Density 1.3 g/cc Rockwell M 99 Rockwell R 126 TensileStrength 97 Mpa Modulus of Elasticity 3.5 Gpa Flexural Modulus 4.1 Gpa

FIG. 5 is a block diagram showing the basic steps of the method oflaterally inserting the implant 100. First the spine is exposed througha lateral access 610. The intervertebral disk is then removed laterally620, if necessary. Following, the implant 100 is inserted laterally 630between the adjacent vertebral bodies. Finally, the wound is closed 640.Additionally, the method includes preparing the spine for the implant bycutting channels into the vertebral bodies to accept the keels of theplates. In one embodiment, the method includes assembling the implant byinserting the crossbar spacer between the upper and lower end platesprior to installation. In one embodiment, the upper and lower end platescan be attached individually to the vertebral bodies and then assembledwith the spacer to form the entire implant assembly thereafter.

What has been disclosed herein has been provided for the purposes ofillustration and description. It is not intended to be exhaustive or tolimit what is disclosed to the precise forms described. Manymodifications and variations will be apparent to the practitionerskilled in the art. What is disclosed was chosen and described in orderto best explain the principles and practical application of theembodiments described herein, thereby enabling others skilled in the artto understand the various embodiments and various modifications that aresuited to the particular use contemplated. It is intended that the scopeof what is disclosed be defined by the following claims and theirequivalence.

1. An intervertebral implant comprising: a. an articulating unit adaptedto be inserted between adjacent vertebral bodies, the articulating unithaving an outer perimeter, and having a first arcuate channel therein,the first arcuate channel having ends spaced inwardly from the firstouter perimeter; b. a crossbar spacer in contact with the first arcuatechannel, wherein the crossbar spacer and the first arcuate channel allowthe articulating unit to rotate in association with flexion andextension of at least one of the vertebral bodies, wherein the crossbarspacer includes i. a first spacer beam having a curved outer sidesurface centered about a first longitudinal axis and having a first endand a second end; and ii. a second spacer beam perpendicular to thefirst spacer beam having a curved outer side surface centered about asecond longitudinal axis, the first spacer beam being disposed relativeto the second spacer beam so that the second longitudinal axis is closerto the first end of the first beam than to the second end, the first andsecond spacer beams each respectively extending beyond opposing sides ofeach other; and c. a first keel adapted to mate with a vertebral body,the first keel extending from a first outer surface of the articulatingunit and oriented to be substantially parallel with the first channel.2. The implant of claim 1 wherein the first keel further comprises aplurality of teeth.
 3. The implant of claim 1 further comprising asecond keel extending from a second outer surface of the articulatingunit, the second keel oriented substantially parallel with the firstchannel.
 4. The implant of claim 3 wherein the first keel furthercomprises a plurality of teeth.
 5. The implant of claim 1 wherein thearticulating unit further comprises a second channel therein to receivethe crossbar spacer wherein the crossbar spacer and the second channelallow the articulating unit to rotate in association with lateralbending of at least one of the vertebral bodies.
 6. The implant of claim5 wherein the first spacer beam is in contact with the first channel inthe articulating unit; and the second spacer beam is in contact with thesecond channel.
 7. The implant of claim 1 wherein the first keel isoriented substantially perpendicular to the sagittal plane of thevertebral body.
 8. An implant adapted to be inserted between adjacentvertebral bodies in a spine comprising: a. a first end plate having afirst inner surface having a first outer perimeter, the first innersurface having a first recessed arcuate channel formed therein, thefirst recessed arcuate channel having ends spaced inwardly from thefirst outer perimeter, and the first end plate having a first keel on afirst outer surface, wherein the first keel is oriented substantiallyparallel to the coronal plane of the spine; b. a second end plate havinga second inner surface opposed to the first inner surface, the secondinner surface having a second outer perimeter, the second inner surfacehaving a second recessed arcuate channel formed therein, the secondrecessed arcuate channel having ends spaced inwardly from the secondouter perimeter, and the second end plate having a second keel on asecond outer surface, wherein the second keel is oriented substantiallyparallel to the first keel; and c. a spacer having a first spacer memberin contact with the first inner surface and a second spacer member incontact with the second inner surface and oriented perpendicular to thefirst spacer member, wherein the first spacer member includes a curvedouter side surface centered about a first longitudinal axis and having afirst end and a second end, and the second spacer member includes acurved outer side surface centered about a second longitudinal axis, thefirst spacer member being disposed relative to the second spacer memberso that the second longitudinal axis is closer to the first end of thefirst member than to the second end, the second spacer member extendingbeyond opposing sides of the first spacer member.
 9. An implant adaptedto be inserted into a spine comprising: a. a first end plate having afirst outer surface adapted to be substantially in contact with an uppervertebral body, and having a first inner surface having a first outerperimeter, the first inner surface having a first recessed arcuatechannel formed therein, the first recessed arcuate channel having endsspaced inwardly from the first outer perimeter; b. a second end platehaving a second outer surface adapted to be substantially in contactwith a lower vertebral body, and having a second inner surface having asecond outer perimeter, the second inner surface having a secondrecessed arcuate channel formed therein, the second recessed arcuatechannel having ends spaced inwardly from the second outer perimeter; c.a first keel protruding from the first outer surface and extendinglengthwise between a first lateral end and a second lateral end of thefirst end plate; d. a second keel protruding from the second outersurface and oriented to be substantially parallel to the first keel; ande. a crossbar spacer disposed between the first end plate and the secondend plate, the crossbar spacer including i. a first spacer beam in thefirst arcuate channel having a curved outer side surface centered abouta first longitudinal axis and having a first end and a second end; andii. a second spacer beam perpendicular to the first spacer beam in thesecond arcuate channel having a curved outer side surface centered abouta second longitudinal axis, the first spacer beam being disposedrelative to the second spacer beam so that the second longitudinal axisis closer to the first end of the first beam than to the second end, thefirst and second spacer beams each respectively extending beyondopposing sides of each other.
 10. An implant adapted to be insertedbetween adjacent vertebral bodies in a spine, the implant comprising: a.an articulating unit having an outer surface adapted to be substantiallyin contact with a vertebral body, the articulating unit have first andsecond opposing inner surfaces, the first inner surface having a firstouter perimeter and having a first recessed arcuate channel formedtherein, the first recessed arcuate channel having ends spaced inwardlyfrom the first outer perimeter, the second inner surface having an outerperimeter, and having a second recessed arcuate channel formed therein,the second recessed arcuate channel having ends spaced inwardly from thesecond outer perimeter; b. a crossbar spacer positioned within thechannels of the articulating unit, wherein the crossbar spacer allows atleast a portion of the articulating unit to rotate in association withflexion and extension of at least one of the vertebral bodies, whereinthe crossbar spacer includes i. a first spacer beam having a curvedouter side surface centered about a first longitudinal axis, and havinga first end and a second end; and ii. a second spacer beam perpendicularto the first spacer beam having a curved outer side surface centeredabout a second longitudinal axis, the first spacer beam being disposedrelative to the second spacer beam so that the second longitudinal axisis closer to the first end of the first beam than to the second end, thefirst and second spacer beams each respectively extending beyondopposing sides of each other; and c. a keel extending from the outersurface of the articulating unit and oriented to extend lengthwisebetween a first lateral end and a second lateral end of the articulatingunit.
 11. An implant adapted to be inserted between adjacent vertebralbodies in a spine, the implant comprising: a. an articulating unithaving an outer surface adapted to be substantially in contact with avertebral body, the articulating unit have first and second opposinginner surfaces, the first inner surface having a first outer perimeterand having a first recessed arcuate channel formed therein, the firstrecessed arcuate channel having ends spaced inwardly from the firstouter perimeter, the second inner surface having an outer penmeter, andhaving a second recessed arcuate channel formed therein, the secondrecessed arcuate channel having ends spaced inwardly from the secondouter perimeter, the channels of the articulating unit having a crossbarspacer therein, wherein the crossbar spacer includes i. a first spacerbeam having a curved outer side surface centered about a firstlongitudinal axis, and having a first end and a second end; and ii. asecond spacer beam perpendicular to the first spacer beam having acurved outer side surface centered about a second longitudinal axis, thefirst spacer beam being disposed relative to the second spacer beam sothat the second longitudinal axis is closer to the first end of thefirst beam than to the second end, the first and second spacer beamseach respectively extending beyond opposing sides of each other; and b.a keel extending from the outer surface of the articulating unit andoriented to extend lengthwise between a first lateral end and a secondlateral end of the unit.
 12. An interspinous disk replacement implant,comprising: an articulating unit having an outer surface adapted to besubstantially in contact with a vertebral body, and having inwardlyfacing inner surfaces, each having an arcuate channel formed therein,the arcuate channels each having ends spaced inwardly from the firstouter perimeter; a crossbar spacer positioned within the channels of thearticulating unit, wherein the crossbar spacer includes a first spacerbeam and a second spacer beam perpendicular to the first spacer beam,the first and second spacer beams being circular in cross-section,wherein the first spacer beam includes a first end and a second end andis disposed relative to the second spacer beam so that the secondlongitudinal axis is closer to the first end of the first beam than tothe second end, the first and second spacer beams each respectivelyextending beyond opposing sides of each other; and at least one keelextending from the articulating unit.
 13. In an interspinous diskreplacement implant in a spine, the improvement comprising: a. anarticulating unit have first and second opposing inner surfaces, thefirst inner surface having a first outer perimeter and having a firstrecessed arcuate channel formed therein, the first recessed arcuatechannel having ends spaced inwardly from the first outer perimeter, thesecond inner surface having an outer perimeter, and having a secondrecessed arcuate channel formed therein, the second recessed arcuatechannel having ends spaced inwardly from the second outer perimeter; b.a crossbar spacer in contact with an inner surface the implant, whereinthe crossbar spacer accommodates flexion and extension movement of thespine, wherein the crossbar spacer includes i. a first spacer beamhaving a curved outer side surface centered about a first longitudinalaxis, and having a first end and a second end; and ii. a second spacerbeam perpendicular to the first spacer beam having a curved outer sidesurface centered about a second longitudinal axis, the first spacer beambeing disposed relative to the second spacer beam so that the secondlongitudinal axis is closer to the first end of the first beam than tothe second end, the first and second spacer beams each respectivelyextending beyond opposing sides of each other; and c. a keel extendingfrom an outer surface of the implant and adapted to be secured into avertebral body, wherein the keel is approximately perpendicular to thesagittal plane of the spine.
 14. An interspinous disk replacementimplant, comprising: an articulating unit having an outer surfaceadapted to be substantially in contact with a vertebral body, and havinginwardly facing inner surfaces, each having an arcuate channel formedtherein, the arcuate channels each having ends spaced inwardly from thefirst outer perimeter; a crossbar spacer positioned within the arcuatechannels of the articulating unit, wherein the crossbar spacer includesa first spacer beam and a second spacer beam perpendicular to the firstspacer beam, the first and second spacer beams being circular incross-section, wherein the first spacer beam includes a first end and asecond end and is disposed relative to the second spacer beam so thatthe second longitudinal axis is closer to the first end of the firstbeam than to the second end, the first and second spacer beams eachrespectively extending beyond opposing sides of each other; and at leastone keel extending from an outer surface of the implant from a firstside to a second side, the at least one keel adapted to be insertableinto the spine.
 15. An intervertebral implant adapted to be insertedbetween adjacent vertebral bodies comprising: a. a first end plateadapted to mate with a first vertebral body, the first end plate furthercomprising: i. a first inner surface having a first outer perimeter, thefirst inner surface having a first recessed arcuate channel formedtherein, the first recessed arcuate channel having ends spaced inwardlyfrom the first outer perimeter; ii. a first outer surface; and iii. afirst keel on the first outer surface, wherein the first keel isoriented substantially perpendicular to the sagittal plane of thevertebral bodies; b. a second end plate having a second outer surfaceadapted to mate with a second vertebral body, the second end plateincluding a second inner surface opposed to the first inner surface andhaving a second outer perimeter, the second inner surface having asecond recessed arcuate channel formed therein, the second recessedarcuate channel having ends spaced inwardly from the second outerperimeter; and c. a crossbar spacer positioned between the first andsecond inner surfaces in the arcuate channels, wherein the crossbarspacer includes i. a first spacer beam having a curved outer sidesurface centered about a first longitudinal axis and having a first endand a second end; and ii. a second spacer beam perpendicular to thefirst spacer beam having a curved outer side surface centered about asecond longitudinal axis, the first spacer beam being disposed relativeto the second spacer beam so that the second longitudinal axis is closerto the first end of the first beam than to the second end, the first andsecond spacer beams each respectively extending beyond opposing sides ofeach other.
 16. The implant of claim 15 wherein the first keel furthercomprises a plurality of angled teeth.
 17. The implant of claim 15wherein the second end plate further comprises a second keel on thesecond outer surface, wherein the second keel is substantiallyperpendicular to the sagittal plane of the vertebral bodies.
 18. Theimplant of claim 17 wherein the second keel further comprises aplurality of angled teeth.
 19. The implant of claim 15 wherein the firstkeel further comprises a plurality of keels on the first outer surface.20. The implant of claim 15 wherein the first spacer beam is in contactwith the first inner surface; and the second spacer beam is in contactwith the second inner surface.
 21. The implant of claim 15 wherein thefirst channel is oriented in a direction extending between an anteriorend and a posterior end of the first end plate.
 22. The implant of claim15 wherein the first channel is oriented in a direction between a firstlateral end and a second lateral end of the first end plate.
 23. Theimplant of claim 15 wherein the second channel is oriented in adirection extending between an anterior end and a posterior end of thesecond end plate.
 24. The implant of claim 15 wherein the second channelis oriented in a direction between a first lateral end and a secondlateral end of the second end plate.
 25. An implant adapted to beinserted between adjacent vertebral bodies, comprising: a. a first endplate having a first outer surface adapted to mate with a firstvertebral body, the first end plate further comprising: i. a first innersurface having a first outer perimeter, the first inner surface having afirst recessed arcuate channel formed therein, the first recessedarcuate channel having ends spaced inwardly from the first outerperimeter; and ii. a first keel on the first outer surface, wherein thefirst keel is oriented substantially perpendicular to the sagittal planeof the vertebral bodies; b. a second end plate having a second outersurface adapted to mate with a second vertebral body, the second endplate further comprising: i. a second inner surface opposed to the firstinner surface, the second inner surface having a second outer perimeter,the second inner surface having a second recessed arcuate channel formedtherein, the second recessed arcuate channel having ends spaced inwardlyfrom the second outer perimeter; and ii. a second keel on the secondouter surface, wherein the second keel is oriented substantiallyperpendicular to the sagittal plane of the vertebral bodies; and c. acrossbar spacer in contact with the first and second inner surfaces,wherein the crossbar spacer includes i. a first spacer beam having acurved outer side surface centered about a first longitudinal axis andhaving a first end and a second end; and ii. a second spacer beamperpendicular to the first spacer beam having a curved outer sidesurface centered about a second longitudinal axis, the first spacer beambeing disposed relative to the second spacer beam so that the secondlongitudinal axis is closer to the first end of the first beam than tothe second end, the first and second spacer beams each respectivelyextending beyond opposing sides of each other.
 26. The implant of claim25 wherein the first keel further comprises a plurality of first keelson the first outer surface.
 27. The implant of claim 25 wherein thefirst keel further comprises a plurality of teeth.
 28. The implant ofclaim 25 wherein the second keel further comprises a plurality of secondkeels on the second outer surface.
 29. The implant of claim 25 whereinthe second keel further comprises a plurality of teeth.
 30. The implantof claim 25 wherein the first spacer beam is in contact with the firstinner surface; and the second spacer beam is in contact with the secondinner surface.
 31. The implant of claim 25 wherein the first channel isoriented in a direction extending between an anterior end and aposterior end of the first end plate.
 32. The implant of claim 25wherein the first channel is oriented in a direction extending between afirst lateral end and a second lateral end of the first end plate. 33.The implant of claim 25 wherein the second channel is oriented in adirection extending between an anterior end and a posterior end of thesecond end plate.
 34. The implant of claim 25 wherein the second channelis oriented in a direction between a first lateral end and a secondlateral end of the second end plate.
 35. An intervertebral implantadapted to be inserted between adjacent vertebral bodies comprising: a.an articulating unit further comprising: i. an anterior end; ii. aposterior end; iii. a first lateral end; iv. a second lateral end; v. afirst inner surface having a first outer perimeter, the first innersurface having a first recessed arcuate channel formed therein, thefirst recessed arcuate channel having ends spaced inwardly from thefirst outer perimeter; and vi. a second outer perimeter, the secondinner surface having a second recessed arcuate channel formed therein,the second recessed arcuate channel having ends spaced inwardly from thesecond outer perimeter; b. a keel extending from the articulating unitand oriented in a lengthwise direction between the first and secondlateral ends; and c. a crossbar spacer positioned within the first andsecond channels of the articulating unit, wherein the crossbar spacerincludes i. a first spacer beam having a curved outer side surfacecentered about a first longitudinal axis, and having a first end and asecond end; and ii. a second spacer beam perpendicular to the firstspacer beam having a curved outer side surface centered about a secondlongitudinal axis, the first spacer beam being disposed relative to thesecond spacer beam so that the second longitudinal axis is closer to thefirst end of the first beam than to the second end, the first and secondspacer beams each respectively extending beyond opposing sides of eachother.
 36. The implant of claim 35 wherein the keel further comprises atleast one first keel extending from a first outer surface of thearticulating unit.
 37. The implant of claim 36 wherein the keel furthercomprises at least one second keel extending from a second outer surfaceof the articulating unit.
 38. The implant of claim 35 wherein the firstspacer beam is in contact with a first inner surface of the articulatingunit; and the second spacer beam is in contact with a second opposedinner surface of the articulating unit.
 39. The implant of claim 35wherein the first channel is oriented lengthwise between the first andsecond lateral ends.
 40. The implant of claim 35 wherein the firstchannel is oriented lengthwise between the anterior and posterior ends.41. The implant of claim 35 wherein the second channel is orientedlengthwise between the first and second lateral ends.
 42. The implant ofclaim 35 wherein the second channel is oriented lengthwise between theanterior and posterior ends.